Progress in the performance, multi-functionality, and accessibility of soft electronic materials has allowed for wearable devices with reduced dependency on rigid or inextensible printed circuit boards. These advances have led to the emergence of deformable circuits and sensors that avoid mechanical incompatibility with skin by matching the elastic properties of soft biological tissue. Applications include wearable monitoring of physiological signals, electronic skin for data entry, and skin-mounted sensors for joint proprioception and motion capture. The ability for these electronics to bend, twist, and stretch is accomplished by using soft elastomers as a carrier medium for deterministically-patterned metal wiring, percolating networks of conductive nanoparticles, grafted electrically active ionomers and conductive polymer groups, and conductive fluids such as carbon filled grease, liquid metals, and ionic liquids. While promising, efforts to incorporate these materials into fully-integrated wearable devices are currently limited by the lack of robust and size-scalable rapid prototyping techniques. Current fabrication methods typically require customized equipment or clean-room fabrication, can be labor-intensive, and take hours to days to create fully functional devices. Such constraints can limit personalized configurations and slow down design iterations, which can be particularly limiting in creating devices for multiple users. These various drawbacks make it difficult to accelerate development and can be potential barriers for scalability and commercialization, especially in the emerging area of personalized health monitoring.
Robust integration of soft materials into wearable computing and bio-monitoring systems requires a comprehensive fabrication approach that allows for a broad range of electronic materials, substrates, circuit architectures, and surface-mounted technologies. This includes reliable methods for patterning, precision alignment, bonding, encapsulation, and electrical interfacing. The latter is particularly essential for local computation and communication, energy-efficient wireless networks, and connectivity with external hardware for power or signal processing. Advances in rapid prototyping techniques such as 3D printing and laser machining of soft materials provide paths forward to achieve these objectives while accelerating design cycles and commercial development. Recent work has shown how these approaches can enable the fabrication of multiple sensors at once with soft materials. However, methods like 3D printing are limited in the mechanical and electrical performance of their build materials and do not support automated integration of IC components, which are necessary for fast, energy efficient, and miniaturized signal processing and communication. This is especially limiting for applications in wearable physiological sensing and bio-monitoring that rely on packaged microchips for biosignal acquisition and processing. Salient examples of this include photoplethysmography (PPG), peripheral capillary oxygen saturation (SpO2) detection, and other modalities that involve optical biosensing. Therefore, a key challenge in wearable bio-monitoring remains the complete and sequential integration of soft sensors, stretchable electrical interconnects, and miniaturized hardware for communication, power, and signal processing through scalable techniques for patterning, encapsulation, bonding, and attachment to soft carrier films.